The control of underwater fluid extraction wells, for example subsea hydrocarbon production wells, is managed by a Subsea Electronics Module (SEM) which is typically housed in a Subsea Control Module (SCM). This is turn is conventionally mounted on a subsea Xmas tree located on the sea bed, above the fluid extraction well. Existing SEMs contain a number of printed wiring boards which perform dedicated functions, such as the operation of hydraulic Directional Control Valves (DCVs). The body of the SEM is typically a metal cylinder of circular cross-section designed to handle the substantial pressure of the environment. This houses well control printed circuit electronic boards, located on connectors mounted on a motherboard, which facilitates connections to input and output connectors at the end of the module as well as the feeding of power supplies from a power supply unit which is typically mounted at the other end of the module. Modern SEMs utilise Ethernet communication between the boards, which requires Ethernet switches to be mounted on each circuit board taking up valuable board area. Because the boards are rectangular, as the most cost effective arrangement, typically the space between the top of the boards and the cylindrical SEM is wasted. Furthermore, there is little space left for any additional components which may be needed, for example control circuitry. In addition, known systems suffer from crosstalk between the Ethernet communications components and the input/output and power feeds, due to the provision of communication electrical connections on the motherboard.